Gas chromatographs are used to determine the chemical composition of a sample, which may be gaseous or a vaporized liquid. The term gas will hereinafter be used to include a vapor. In one type of gas chromatograph, a sample is sent through a separation column. A typical separation column is a long capillary tube with a coated interior. Different chemical compounds in the sample travel through the separation column at different rates and leave the separation column at different times. As compounds leave the separation column, they are carried by a carrier gas past a detector. The detector detects compounds in the carrier gas by measuring changes in the properties of the effluent gas. When a change in the gas property occurs, the timing of the change indicates the type of the compound passing the detector, and the magnitude of the change indicates the quantity of the compound.
One type of detector used with gas chromatographs is a thermal conductivity detector, which detects changes in the thermal conductivity of the effluent gas. When a compound is mixed with the carrier gas, the thermal conductivity of the mixture is usually different from that of the pure carrier gas. A thermal conductivity detector provides a measure of the change in the thermal conductivity of the carrier gas and thereby provides a measure of the presence and amount of various compounds.
FIG. 1 shows a typical prior art sensor circuit 10 used in a thermal conductivity detector of a gas chromatograph. The sensor circuit 10 includes a metal filament 22, such as a platinum wire, placed in a wall 18 of a cavity 24. The effluent from a gas separation column along with a carrier gas fills the cavity 24 and flows past the filament 22. The filament 22 has a resistance R.sub.S which depends on its temperature and is heated using an electric current I.sub.2. In the case of the filament 22 being a platinum wire, the resistance of the filament 22 is proportional to its temperature.
Heat generated by the filament 22 is removed partially by the flow of the effluent but primarily by thermal conduction through the gas to the walls 18 of the cavity 24, thus lowering the resistance of the filament 22. By effectively measuring the change in resistance of the heated filament 22, the change in thermal conductivity of the flowing gas may be determined.
In some applications, problems arise that can cause the output of a detector to change even if the composition of the gas remains constant. One problem is caused by changes in the temperature of the walls 18 of the cavity 24. Another problem is caused by changes in the temperature of the carrier gas. Another problem is electronic drift of the energizing voltage which controls the current through the filament 22. With the sensitivity required of a detector, even thermoelectric potentials generated in electrical connections may affect the detector. Changes in the voltage offset of the amplifiers used to measure changes in the resistance of filament 22 are still another problem.
Thus, one of the problems encountered with such detectors is that the heat flow between the filament and the cavity wall 18 of the cavity 24 is directly affected by the temperature of the cavity wall 18. For this reason it has been customary to reduce the effect of ambient temperatures on the temperature of the cavity wall 18 by imbedding the cavity 24 in a heated block 14. A temperature control circuit (not shown) is employed to heat the block to an elevated temperature, such as approximately 150 to 250 degrees C. The block temperature is typically selected by the user according to the requirements of the particular type of analysis being performed.
The filament 22 is operated in a bridge circuit employing an embedded resistor 12 that is typically a platinum resistance type (PRT) sensor and is located in the cavity wall 18 of cavity 24. Resistors R.sub.b and R.sub.a have fixed resistance values and are chosen to give the desired resistance in filament 22 when the bridge circuit is balanced. A differential amplifier 26 detects an unbalance in the bridge. A DC voltage (V.sub.A) is used to heat the filament 22 to a temperature typically between 40 degree(s) to 100 degree(s) C. above the temperature of the cavity wall 18 in accordance with balanced operation of the bridge circuit. If the thermal conductivity of the effluent in the cavity 24 is different from that of a pure carrier gas in cavity 24, the bridge becomes unbalanced, and a change in the amplifier's output voltage V.sub.A indicates the detection of a change in thermal conductivity of the gas in the cavity 24. A variation in the wall temperature of the cavity 24 will change the resistance of the embedded resistor 12 and thus alter the balance point of the bridge circuit. The power supplied to filament 22 thereby changes, thus altering the temperature of the filament 22 to compensate for the effects of a variation in the cavity wall temperature.
If the temperature of the filament 22 is not set to exceed the temperature of the cavity wall 18 by a sufficient temperature differential, the sensitivity of the sensor circuit 10 suffers and the output signal V.sub.A may be subject to the effects of noise signals. If the temperature differential is too large, then the filament 22 expands and may no longer be kept under tension. The output signal V.sub.A may then be subject to the effects of noise signals and the filament 22 is subject to destructive contact with the cavity wall 18. It would be advantageous, therefore, to provide a thermal conductivity detector that provides an appropriate temperature differential in an automatic, accurate, and reliable fashion.
Another problem is that the embedded resistor 12 offers less than satisfactory performance in some applications. The resistance or temperature coefficient of the embedded resistor 12 may change with time, thus causing an undesirable shift in the temperature differential between the temperature of the cell wall 18 and the temperature of the filament 22. The embedded resistor 12 is expensive, and must be carefully and properly embedded in the block 14, thus increasing manufacturing costs. The embedded resistor 12 also presents a problem in that its leads must be directed out of the block 14 so as to be attached to the appropriate connections in the sensor circuit 10. These leads are prone to failure because they are fragile and the insulation material on the leads is subject to degradation at the elevated temperatures of the block 14 when heated. The insulation can fail and cause a short circuit between a lead and the block 14. A further problem occurs when the leads from the embedded resistor 12 are accidentally confused with the leads from the filament 22 during assembly of the circuit 10. The resulting connections can allow misdirection of current in the circuit 10, thus damaging certain components in the circuit.
Accordingly, an improved sensing circuit and method of operation for detecting thermal conductivity are needed to avoid the problems associated with the embedded resistor 12 in a bridge type sensor circuit 10.